OFDM versus traditional narrowband communications


Rudimentary powerline communication (PLC) schemes have been used for many decades. However, new “smart grid” applications and advanced metering infrastructures (AMI) require more frequent and reliable communications than possible with basic PLC methods. The new MAX2990 enables a reliable PLC data rate of up to 100 kbps effectively over a wide, 10 kHz to 490 kHz frequency range.

Powerlines Can Be Hostile Environments
Powerline channel characteristics and parameters vary with frequency, location, time, and equipment. The lower frequency regions, from 10 kHz to 500 kHz, are especially susceptible to interference. Moreover, the powerline is a very frequency-selective channel. Aside from background noise, it is subject to impulsive noise that often occurs at 50 Hz or 60 Hz, and narrowband interference or group delays of up to several hundred microseconds.

Radio specifications

Figure 1 – OFDM (top) provides better transmission in the CENELEC A band than narrowband (bottom)

Traditional narrowband modulation schemes, such as FSK, cannot withstand group delay and narrowband interference, so data can get lost. Narrowband transmissions are unreliable under harsh conditions, and a higher data rate may not be possible using traditional methods.

OFDM enables reliable transmission and improved data rates
Orthogonal frequency division multiplexing (OFDM) is a modulation technique that can utilise bandwidth very efficiently, thus allowing the use of advanced channel coding techniques; this combination enables very robust communication in the presence of narrowband interference, impulsive noise, and frequency-selective attenuation. The graph in Figure 1 illustrates why OFDM is so much better than narrowband for data communications. For this example, eight tones between 10 kHz and 95 kHz are used, providing a useable channel bandwidth of 85 kHz. In contrast, a narrowband solution uses only two tones to transmit data in that bandwidth.

In both cases, 4 data bits and 4 error correction bits are sent. On the top half, we see that OFDM can transmit all 8 bits with a single symbol. On the bottom, we can see that the narrowband needs four symbols to transmit the same payload. Because OFDM uses the spectrum more efficiently, it opens the channel for more data, and thereby a higher data rate.

M-Bus module

Forward error correction plus OFDM increases reliability
OFDM is one of the principle methods used in the new MAX2990 modem. Additionally, both convolutional and Reed- Solomon coding have been integrated to provide redundancy bits. This allows the receiver to recover bits lost due to background and impulsive noise. An interleaving scheme is also used to decrease the correlation of received noise at the input of the decoder.

A typical narrowband FSK modem in the CENELEC A band can only transmit 2 kbps at 12 dB SNR (the signal is four times stronger than noise) with a bit error rate (BER) of 10-4. This means that 1 bit in 10,000 transmitted is lost. However, an OFDM system in the CENELEC A band can transmit up to 32 kbps, and it does this at around 4 dB SNR.

Robust mode ensures reliable communication
Sometimes, interference on the line can block tones in certain frequencies. When line conditions degrade in that manner, the MAX2990 switches to a robust mode. In this mode, data is repeated several times, with each repetition utilising the convolutional coding and Reed-Solomon encoder. This robust mode provides an additional 5 dB of reliability in data PLC.

Wide frequency range complies with international comunication standards.
Various parts of the world have their own unique regulatory constraints governing the use of powerline signaling. Europe, Japan, and the US each allocate different bands for powerline communications, as governed by CENELEC, ARIB, and the FCC, respectively. Supporting a wide, 10 kHz to 490 kHz frequency range, the MAX2990 can be programmed to comply with all of these standards, thus allowing a single modem design to be used worldwide.

OFDM dramatically increases the amount and reliability of data that can be sent over a powerline. The MAX2990 combines OFDM with other advanced techniques to optimise the communication potential of power grids worldwide.